Regularized Orbital-Optimized Møller-Plesset Perturbation Theory for Strongly Correlated Species

  • Unrestricted Hartree-Fock (UHF); may exhibit artificial spin-symmetry-breaking in open-shell systems, and using UHF orbitals for correlated wave function calculations could lead to incorrect energetics and properties.
  • Orbital-optimized MP2 (OOMP2) restores spin-symmetry and remove the discontinuity in the first-order properties at the onset of symmetry-breaking in MP2.
  • Orbital optimization in OOMP2 could produce very small energy denominators which make the total energy diverge. In addition, OOMP2 may unphysically prefer restricted solutions and remove the Coulson-Fischer point at potential energy curves.
  • Q-Chem offers several regularization schemes (κ-OOMP2 and σ-OOMP2) to fix the issues above. Combined with approximate spin-projection, these regularized OOMP2 methods are powerful as well as economical tools in studying strongly correlated systems.

C30(D5h symmetry)


Measures of symmetry breaking parameters as functions of the regularization strength of C30